If you think your new co-workers will be watching you like a hawk, you’re right. Don’t give them fodder for the gossip mill.

YOUR first day at a new job; it’s like the first day of school all over again. The only difference is that you’ve traded your backpack for Balenciaga, the calc-ulator for a shiny new BlackBerry, and a sparkling pair of kicks for those gorgeous, grown-up pumps or loafers. Sure, you’ve grown in age since you first experienced those “I’m finally going to middle school” butterflies – but the sick feeling in your stomach on the first day of a new job feels familiar.

Like school, a new professional gig is all about impressing the older kids, trying not to fall flat on your face while juggling a tray in the cafeteria – and, most of all, blending in just enough so they don’t think you’re a total freak. “The first 90 days are the most critical in any new job,” says Nancy Friedberg,president of Career Leverage, a Manhattan-based executive coaching and career-management firm. “During that time period, the most important thing to do is to observe, listen and learn.”

According to Friedberg, the worst mistake any new kid in the Cubicle Neighborhood can make is to show up with guns blazing, ready to take over the company. She suggests taking time to observe your new environment, paying special attention to how people interact with one another. “You should never, ever try to implement any major changes that could be perceived as radical,” she says. “And relationships are the most important thing to observe in the first three months.”

This means it’s essential to watch and learn. No one is going to give you a rundown of the social inner workings of the office crew, so you need to put your powers of observation to the test.

“Look for who has power and influence. Who is the most influential in terms of making and carrying out decisions, and who is the inner circle? Don’t befriend anyone too quickly, because you don’t know if they’re politically ‘out’ or not. And don’t be too goal-oriented or too task-focused; try and get a feel for the whole rather than small parts.”

Denise, a 24-year-old public relations professional, learned the hard way how important alliances can be – she made the wrong one and spent a lot of time cleaning up the mess.

“I was the new kid, so when Brooke was so nice to me the first few days, I was totally relieved,” she says. “I was this wide-eyed kid and she was this older, more experienced person who was willing to show me the ropes,” she says.

But after a couple of weeks, Denise realized that while she and Brooke were growing closer, no one else in the office seemed to want to have anything to do with her. They were all nice and courteous, but there were no invitations to join them for lunch or after-work cocktails.

“It was like I had body odor, the plague and a face full of contagious boils,” she says. “And then it hit me – it wasn’t me. It was Brooke; she was the most hated woman in the office.”

Denise quickly ended things with Brooke, politely as possible. But the damage was already done. She says it was almost impossible for people to get over their initial impressions of her, which were mostly formed in observing her alliance with Brooke.

“It was really tough trying to convince them – without saying anything too openly – that I realized what a total tool I had been getting into bed with someone so quickly in to the game,” she says.

But fitting in can be even harder if you’re filling in for someone who got demoted, or if the person who left was the most popular person in the office. Burkat suggests maintaining your professionalism and staying away from condescension and gloating. If you filled a spot that was vacated because the person wasn’t capable, be gracious – especially if the person still works there.

If, on the other hand, you’re filling Mr. or Ms. Popular’s spot, try not to get too intimidated by the legacy they left behind.

“Taking over for someone who’s been demoted – or someone who was the golden kid – requires a strong resolve and cautious going. Maintain your professionalism and avoid any condescension,” says Burkat. “Continued surliness and hostility on the demoted person’s part won’t do much to please management, which is your goal.”

Many people are instantly hostile to any new office blood. Burkat says you’re there to develop your career, not make friends. And while casual and friendly conver-sations with co-workers make for a more comfortable work atmosphere, your primary goal at any new job is to prove that you were a good hire.

“Being new to a company is always going to take some getting used to; our motto is ‘Head down, antenna up,'” she says.

“But if your co-workers aren’t friendly first, don’t fret; some people take time to warm up.”

Anthony Grillo, a career coach for Life in Motion Coaching, has seen worse things than bad attitudes happen on the first day.

“I had a client who got a great offer from a company, accepted the job and found out that the owner was indicted and the company closed, all on their first day,” he says.

“And I had another client who got hired and showed up for their first day to find out that the person who hired them had been fired and their position was no longer available.”

For the less extreme first days, Grillo suggests showing up to your new job on time and ready to immerse yourself in your new surroundings.

“Ask questions; any new hires are afraid to do this on the first day. They just listen in hopes of figuring things out on their own,” he says.

“Also, accept offers from co-workers to go to lunch; start joining your new team ASAP.”

According to Friedberg, it’s important to ask questions as well as set realistic 30-, 60- and 90-day goals for you with your boss.

“This is the only way to know that you’re on the right track, and it will help manage your expectations as well as those of your boss,” she says.

“It’s important to find out how you can best support your boss and respond to their requests immediately.”

Questions are key – as long as they’re appropriate.

Now is not the time to ask for more vacation or dig for dirt on interoffice affairs, past and present. But, even though you’re new, and you’ve got a ton of questions you’re dying to get answered, don’t become a permanent fixture in your boss’ office. There is a fine line between an inquiring, inquisitive new employee and an annoying, won’t-stop-talking new person.

“Ask questions that are relevant to the task at hand; avoid vagaries and focus on specific aspects of what needs to be accomplished,” says Burkat.

“Keep a list of daily things that need to be discussed and, if possible, set a specific time during which you talk with the boss.”

For most people, the first few days offer their first chance at making good impressions on superiors and co-workers, so try to avoid these usual pitfalls. “Don’t ask if it’s OK to leave or rush out at quitting times. First days are usually not as stressful or long as others, but wait until your boss or manager gives you the OK to clock out,” says Grillo.

“And always ask if there’s anything that needs to be done before leaving.”

He also advises to avoid checking personal e-mail, surfing the Web or making and answering calls on your cellphone. And please don’t forget to turn your cellphone on vibrate or silent; above all, your new co-workers don’t need to know about your secret L.F.O. obsession via your ringtone.

TOP 10 things to avoid your first day

* Eating anything pungent at your desk for lunch. Leave the fish curry at home, please – unless you want to be known as “El Stinko” right off the bat.

* If the baby sitter cancels, do not bring your kid to work.

* It’s OK if your boyfriend/girlfriend walks you to work the first day. It is not OK to launch into a full-frontal goodbye in front of the building.

* Don’t talk about planning a vacation, unless you discussed it in your interview.

* Sure, your previous boss was a lazy drunk. But no one at your new company needs to know about that; trash talking is a big no-no.

* Try to avoid talking about politics or anything else that can be inflammatory. You don’t know people well enough to catch them up on your last three weeks of participating in all those anti-war rallies.

* While it’s true that teal blue halter always scores you free drinks at Guest House, it’s not going to win you anything but stares at the office. At work, you want people to respect you professionally – so unless your new job is at Scores, don’t wear anything too revealing or offensive.

* It’s OK to bring in some pictures and trinkets to personalize your work space. However, you might want to leave the 2006 bathing beach beauties calendar at home; it doesn’t fly in the office.

* Don’t lie. The pressure to impress is at its highest the first few days of a new job, but saying you’ve done things that you’ve never experienced, or playing up strengths you don’t possess, will only get you in big trouble in the future.